This invention relates generally to an arrangement for electrically controlling a physical quantity or parameter contained in a rotatable drum from a stationary location external thereto and more specifically to the improved design of a set point transmitter and associated regulator which maintains a desired parameter value in the drum in spite of variations in the resistance of the slip rings used to apply the electrical current and set point control signal to the regulating circuitry contained in the roll. While the invention will be described in connection with its application to a drum type heater, it is to be understood that it may be utilized in other applications.
In prior art drum-type heater arrangements, it is the common practice to mount both the heater regulating circuitry and the set point controller on a common shaft so that the two rotate in unison and power is fed to the combination through slip rings. The simultaneous rotation of the controller and the heat regulator is dictated by the fact that early attempts to transmit the set point information through slip rings to the rotating temperature regulating circuitry has resulted in unworkable devices due to the fact that the set point adjustment has been subject to variations with changes in the resistance of the slip rings and brushes used in the system. Typical of these prior art attempts are the inventions described in the Pettit U.S. Pat. No. 3,290,485, the Scowcroft U.S. Pat. No. 3,286,081, the Hosack U.S. Pat. No. 2,700,094, and the Nordquist U.S. Pat. No. 2,357,845. Each of these patents shows heated rollers having temperature sensors and heating elements mounted in the rollers and having temperature controllers rotating with the roller. In these prior art arrangements, in order to vary the roll temperature, it is necessary to bring the roll to a stop, make the desired adjustment in the heater control set-point mechanism, and then re-start the unit. It is, of course, desirable to be able to vary the temperature setting of the heating elements used in the roll while the roll continues in motion.
In accordance with the teachings of the present invention, a roll heater arrangement is provided in which the final control element rotates with the drum, but the stable reference used for external adjustability is remotely connected to the drum controller through means of a slip ring or rings. The prior art approaches have always resulted in instabilities of temperature control because of variations in the contact resistance of the slip rings and brushes due to wear, contamination, etc. By using the present invention, rather inexpensive slip rings may be employed to convey the set-point control signal to the rotating heater control element. This is accomplished by providing a unique transmitter that provides a constant current signal which is a function of a desired set point to the heater controller, the control signal being relatively unaffected by variations in the impedance introduced by the slip rings.